Recently, semiconductor crystals composed of Group IV elements for use in high-speed semiconductor devices have attracted widespread public attention. Among these, research and development related to semiconductors composed of Group IV elements containing carbon (SiGeC, Si:C (Si crystal containing a few atomic percent substitutional C atoms). Ge:C (Ge crystal containing a few atomic percent substitutional C atoms), etc.) has been actively conducted in recent years (for example, Japanese Unexamined Patent Publication Nos. 2001-196317, 2001-93900, 1999-284065 (Specification of U.S. Pat. No. 6,251,751). This is because these crystals have the following excellent properties.
(1) First, the advantages of the Si:C crystal will be explained. The Si:C crystal has a smaller lattice constant than the Si crystal because the carbon atom is smaller than the silicon atom. Therefore, when epitaxially grown on an Si substrate, Si:C crystals become strained, because they receive tensile stress toward the in-plane direction of the substrate. In strained Si:C crystals, so-called intervalley scattering, which is one of the scattering mechanisms of conduction electrons, is reduced compared to that of bulk Si crystals. The reduced scattering enables Si:C crystals that have been epitaxially grown on an Si substrate to obtain greater mobility than that of bulk Si crystals. Therefore, by applying a heterostructure composed of Si:C crystals and Si crystals, it is possible to achieve a device that operates at higher speed than is possible using only bulk Si crystals.
(2) Next, the SiGeC crystal, which can be considered as an improvement of the SiGe crystal, will be explained.
The SiGe crystal has a larger lattice constant than the Si crystal. Therefore, when an SiGe crystal has been epitaxially grown on an Si substrate, it is subjected to an extremely large compressive stress, generating strain. This compressive strain limits the film thickness to the so-called critical film thickness (the upper limit of film thickness that can be deposited without dislocation), and may lead to relaxation accompanied by crystal defects (dislocations) when the SiGe crystal is additionally subjected to heat treatment. Furthermore, in a heterostructure composed of Si and SiGe crystals, band offset appears only on the valence band of the SiGe crystal. Therefore, when an MOS transistor with an SiGe crystal channel is fabricated, only a p-channel transistor can be obtained.
However, since carbon has a smaller atomic radius than Si or Ge, adding carbon atoms to an SiGe crystal reduces the lattice constant of the crystal and compensates the strain. The critical film thickness can thus be made thicker than that of the SiGe crystal. Furthermore, it is also possible to reduce the amount of strain accumulated in the crystal, thereby raising the thermal resistance of the crystal. From the viewpoint of its application to devices, in a heterostructure composed of SiGeC and Si crystals, when the concentrations of Ge and C are increased (Ge: dozens of %, C: several % or greater), band offsets appear both on the valence band and conduction band of the SiGeC crystal. In this case, carrier trapping occurs in both the conduction band and valence band. Therefore, the use of SiGeC crystal as a channel material is advantageous in that not only p-channel MOS transistors but also n-channel MOS transistors can be fabricated.
As described above, the Si:C crystal and SiGeC crystal have excellent properties, and the additional effects obtained by adding carbon atoms thereto become more distinctive by increasing the concentration of carbon thereof. For example, if the carbon concentration of the Si:C crystal becomes higher, a significant effect on the prevention of scattering can be expected.
However, it is essentially difficult to dissolve a carbon atom in Si or Ge, and therefore a high-quality crystal with a high carbon concentration cannot be readily fabricated. Furthermore, a high carbon concentration in the crystal leads to the following problems.
Carbon atoms tend to be incorporated not only into substitutional lattice sites but also into interstitial lattice sites. The interstitial carbon atoms tend to form a trap with a positive or negative charge in the crystal. In fabricating transistors, having such traps causes the recombination or scattering of carriers, thereby degrading the properties of the device.
In order to obtain high-quality Si:C or SiGeC crystals having a low interstitial carbon concentration, methods known as CVD (Chemical Vapor Deposition) and MBE (Molecular Beam Epitaxy) have been heretofore employed. In these methods, in order to minimize the number of interstitial carbon atoms, various modifications have been made to the crystal growing method (lowering the growth temperature or increasing the total pressure of the source gas).
However, even when these techniques are employed, if the carbon concentration increases to a certain degree, there is a tendency for carbon atoms to be inserted between the lattice positions. Therefore, it was difficult to fabricate a Group IV element semiconductor wherein all the carbon atoms were incorporated into substitutional lattice sites. As a result, the interstitial carbon atoms cause the recombination or scattering of the carriers as described above. Therefore, applying such crystals to a semiconductor device poses a problem because they have a significant effect on the electrical properties.